This review contains spoilers.

If, heaven forfend, I were ever murdered in Barnet, I’d want DS Marcella Backland to investigate. Never mind that odds are she’d probably turn out to have done it, Marcella’s still the best around. Episode six may have poured petrol on the already-raging dumpster fire that is her personal life, but at least it showed her detective-ing it up a storm.

Why Marcella’s spot-on professional instincts don’t apply to her love life is unclear. At work, she sniffs out liars and arrives at the right answer faster than you can say ‘Keith Allen pushed Nigel Planer in the canal’. At home though, she doesn’t notice her husband’s three-year affair, her son’s four-year lie, or her current boyfriend being a cheating, lying twonk in a Topman suit.

She knows all that now, of course, as well as a few other things. One is that the lingering smell in the station isn’t down to that time Techy Mark steamed a bag of Brussel sprouts in the office microwave, it’s because of the decomposing corpse hidden under the lift shaft. Another tampon-based revelation is that Discman is most likely… Discwoman!

To accompany that newsflash, this episode presented the audience with a reformed 90s girl band’s worth of suspects. There’s Maya Whitman, Dr Lewis, Jojo the vet, and in a surprise move, Jason’s fiancée and rehab nurse Becky Marani (the youngest, she must have been the bandmember drafted in after the first solo career departure). Somewhere, we’re led to believe, among this quartet of menstruating blondes—place your bets now that forensics are about to pull a long blonde hair off one of the corpses—is our killer.

For it to be Becky, she’d have to have started her child abduction/pop-up lobotomy clinic around the period she was doing her GCSEs. That doesn’t make it impossible, but with all that revision and coursework, frankly, who has the time? Abducting children and keeping them hostage in a remote farmhouse also requires use of a car and your mum letting you stay out late on weeknights. The jury’s out on her until Techy Mark’s search for blackmail dirt digs up something suspicious.

Until now, Dr Lewis has mostly been employed in a carrying-clipboards-in-the-back-of-shot capacity. She’s been roughly as important to the plot as say, a lamp or coffee table. That all changed this episode with the news that she’s the sister of Debbie Canavan, the teenager buried in the garden of Reg’s flat in the 80s. With the surgical nous to carry out operations and a reason to have a serious grudge against Reg, Lewis could well be our perp. Nothing in her office pointed towards her being a serial killer, though I didn’t get a good look at her desk tidy.

There’s also Maya Whitman, seen this week wearing the tight white jeans of a murderer and keeping a box of the cheap toys with which the victims are buried in the boot of her car. She shares the Kids Call link (the leap Marcella and co. have yet to make is that all the abducted children were being abused before the killer took them) with Jojo the vet, but presumably not her medical skills.

Is Jojo really a suspect? After Vince’s arrest scuppered her chance at getting the money for a seventh IVF attempt, she was prompted to enact Operation Sad Toilet Shag. In a very queasy cubicle scene, the vet went against all her instincts to try to get pregnant, then quickly regretted it. Counter-intuitive as this might seem, Jojo feels too unstable to be our killer. Insanity notwithstanding, to have evaded capture this long, whoever it is sticking drill bits into kids’ brains then stashing their bodies all over North London must be a pretty together sort of person. The vet’s surely too much of a mess.

Also in a bit of a mess this week was Samantha’s mum Jane, who, in parallel with Edward, had lied to the police for years about her best friend’s disappearance. Like Edward, Jane felt guilt and responsibility for her pal’s death, and wondered if she could have saved Debbie had she behaved differently. In the midst of this sensationalist story, there was something sadly convincing about Jane’s account of losing her virginity underage to a famous musician, a scene played by actor Michelle Terry with real emotion.

On the subject of emotion, Angry Eric briefly experienced a second one this week: momentary satisfaction. His blackmail scam with Karim literally paid off, but his steak and chips celebration dinner was followed by a dessert of getting unfortunately kicked in the head. Add to that sister Gail’s apparent disappearance, and episode six couldn’t really be called a win for him.  

Neither could it for our lead. Unless Tom, sorry, Tim, is revealed to be acting as the honeypot in an undercover investigation of Maya Whitman, that’s another failed relationship ending in acrimony. Ah well, if Becky does turn out to have something to do with all this child killing, Jason will at least have to shut up about Marcella being an unfit parent.

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With two episodes to go, this series’ plot tangle is starting to unravel. We’re approaching day twelve of the investigation, when—remember—some fresh horror will provoke Marcella to climb up on that roof ledge. With everything already having gone so wrong for her (“Jason, my kids, my health, Tim…”), how could things possibly get worse? She’s closer than ever to uncovering the killer’s identity.

Speaking of which, come on, Little Adam. Time to wake up out of that coma, love. We need you to look at some photos of blonde women and kick the final chase into gear.

Read Louisa’s review of the previous episode here.